This invention relates to lighting fixtures in general and more particularly to an improved lighting fixture in which the light distribution curve can be simply and effectively changed.
Lighting fixtures are known in which opposed lateral reflectors bounding the light egress opening and enclosing the lamp between them are mounted on support means at an angle of inclination which can not be changed relative to the plane of the light egress opening. In one known fixture the lateral reflectors are arranged such that they can be horizontally adjusted. However, the angle of inclination of the lateral reflectors with respect to the plane of the light egress opening remains unchanged. In another known type of lighting fixture only the angle of inclination is changeable with the lateral reflectors as a whole arranged tiltable about an axis parallel to the longitudinal axis of the lamp. These lighting fixtures are of types designed in particular for street lighting and quite often will be used in conjunction with mercury vapor lamps or the like.
In these known lighting fixtures, however, a change of the basic shape of the light distribution curve, hereinafter the LDC, is not possible. Instead, only the location of the two radiation maxima in the vertical and/or horizontal plane can be changed. It is not possible to increase the light intensity in the region of the minimum between the two radiation maxima.
As a result lighting fixtures of this nature do not allow a lighting installation, such as street lighting installation, to be optimally adjusted to the surface of the street pavement in all cases. The surface structure of the street pavement can vary from a dull surface to a very smooth surface and can contain greatly differing color components. These factors determine the reflecting properties of the pavement which must be matched to the light distribution of a lighting fixture if the most uniformly possible light density distribution is to be achieved. Furthermore, since the reflecting properties of a pavement change with time, corresponding adaptations of the LDC of the light fixture is also desireable. Starting with these considerations in mind, the present invention is based on the fact that for optimum adaptation of the lighting fixture to the pavement conditions in each case, the characteristics of the LDC of the lighting fixture must also be variable between an extremely wide radiation pattern and a nearly circular pattern.
It is known that the two lateral mirrors of a trough-like reflector of three plane elements linked to each other can be constructed. However, a large number of possible adjustments makes it a practical impossibility to utilize such structures for adapting the light distribution curve to the nature of the street surface in each given case since it would require tests and measurement taking a number of days to perform and would still not lead to an optimal result which could be transferred to other light fixtures of the same type. This is particularly true since in this known type of device no interrelationship of any kind between the reflecting properties of the street surface the light distribution curve optimally adapted to such street surface and the adjustment of the reflector elements therefor are known. Furthermore, in this known type of device, as the mirrors are moved, the glare conditions also change.
In view of these problems with prior art apparatus, the need for an improved lighting fixture of the general type described above in which the light distribution curve can be adapted to the different reflecting properties of the street pavement through a simple adjustment in an optimal and reproducible manner becomes evident.